1. Field of the Invention
The present invention relates to a coupling apparatus that couples two objects, an exposure apparatus that exposes a substrate via a projection optical system in a state wherein a liquid is filled between the projection optical system and the substrate, and a device fabricating method that uses the exposure apparatus.
2. Description of Related Art
Semiconductor devices and liquid crystal display devices are fabricated by a so-called photolithography technique, wherein a pattern formed on a mask is transferred onto a photosensitive substrate. An exposure apparatus used by this photolithographic process comprises a mask stage that supports the mask, and a substrate stage that supports the substrate, and transfers the pattern of the mask onto the substrate via a projection optical system while successively moving the mask stage and the substrate stage. There has been demand in recent years for higher resolution projection optical systems in order to handle the much higher levels of integration of device patterns. As the exposure wavelength to be used is shorter, the resolution of the projection optical system becomes higher. As the numerical aperture of the projection optical system is larger, the resolution of the projection optical system becomes higher. Consequently, the exposure wavelength used in exposure apparatuses has shortened year by year, and the numerical aperture of projection optical systems has also increased. Furthermore, the currently mainstream exposure wavelength is the 248 nm KrF excimer laser, but an even shorter wavelength 193 nm ArF excimer laser is also being commercialized. In addition, the depth of focus (DOF) is also important as well as the resolution when performing an exposure. The following equations respectively express the resolution R and the depth of focus δ.R=k1·λ/NA,  (1)δ=±k2·λ/NA2,  (2)
Therein, λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are the process coefficients. Equations (1) and (2) teach that shortening the exposure wavelength λ increases the resolution R, and that increasing the numerical aperture NA decreases the depth of focus δ.
If the depth of focus δ becomes excessively small, then it will become difficult to align the surface of the substrate with the image plane of the projection optical system, and there will be a risk of insufficient margin of focus during the exposure operation. Accordingly, a liquid immersion method has been proposed, as disclosed in, for example, PCT International Publication No. WO99/49504 (Patent Document 1), as a method to substantially shorten the exposure wavelength and increase the depth of focus. In this liquid immersion method, the space between the lower surface of the projection optical system and the substrate surface is filled with a liquid such as water or any organic solvent to utilize the fact that the wavelength of the exposure light beam in the liquid is 1/n as compared with that in the air (n represents the refractive index of the liquid, which is about 1.2 to 1.6 in ordinary cases) so that the resolution is improved and the depth of focus is magnified about n times. Furthermore, the contents of the above-mentioned Patent Document 1 are hereby incorporated by reference in its entirety to the extent permitted by the laws or regulations of the states designated or elected by the present international patent application.
Incidentally, there is a possibility that vibrations produced by the movement of the substrate stage that holds the substrate, and the like, in the state wherein the liquid is filled between the end surface (the terminal end surface) of the optical member on the most substrate side of the projection optical system and the substrate surface, will transmit to the optical member of the terminal end thereof via the liquid, and that the pattern image projected onto the substrate via the projection optical system and the liquid will unfortunately degrade. In addition, there is a possibility that changes in the pressure of that liquid will apply force to the projection optical system, and will fluctuate the projection optical system, and unfortunately degrade the pattern image projected onto the substrate.